Sell Twitter Followers

I really have been thinking about the Fiverr thing lately and I really want to know how they do it. I have been searching high and low for this and I cant seem to find much at all.
Please help me!

you mean u want to know how i can do this gig ? http://bit.ly/MofdmG

the seller are buying xk accounts and then they are adding the accounts via a php script to the accounts from their customers.

they can add every account to 2000 + x customer accounts

THE risk if the accounts get caught as fake or spam accounts by twitter:
the seller must/should add new followers to the seller accounts.

The sellers don't have to be buying the accounts, they can create new accounts themselves. Twitter is ridiculously easy to automate. They don't even make you do a captcha if you create the account with a fresh proxy (and no cookies), if memory serves. Just start reading the threads about the different programs that are out there to automate account creations/posting, and see which suits you best.

you will need accounts and twitter software + good proxies to do that